M18 Has No Semi

I recently aquired 4 glocks. 1 is just the shell, the other is a M17, and the other two are M18s. Now for my problem. One of M18s fire full auto on semi. The second one is more of a parts guns so I have plenty of parts to switch around. I checked everything and didn't notice anything worn or scratched up really bad. I have read all the old stuff I could find but all I found totaled a grand total of 2 threads. The only thing I haven't tried is shimming the slide because I don't know how to do that. How would I tell if that is what needs to be done and how do I do that?